Active electrode material

ABSTRACT

The invention relates to active electrode materials and to methods for the manufacture of active electrode materials. Such materials are of interest as active electrode materials in lithium-ion or sodium-ion batteries. The invention provides an active electrode material expressed by the general formula M1aM22-aM3bNb34-bO87-c-dQd.

FIELD OF THE INVENTION

The present invention relates to active electrode materials and tomethods for the manufacture of active electrode materials. Suchmaterials are of interest as active electrode materials in lithium-ionor sodium-ion batteries, for example as anode materials for lithium-ionbatteries.

BACKGROUND

Lithium-ion (Li-ion) batteries are a commonly used type of rechargeablebattery with a global market predicted to grow to $200bn by 2030. Li-ionbatteries are the technology of choice for electric vehicles that havemultiple demands across technical performance to environmental impact,providing a viable pathway for a green automotive industry.

A typical lithium-ion battery is composed of multiple cells connected inseries or in parallel. Each individual cell is usually composed of ananode (negative polarity electrode) and a cathode (positive polarityelectrode), separated by a porous, electrically insulating membrane(called a separator), immersed into a liquid (called an electrolyte)enabling lithium ions transport.

In most systems, the electrodes are composed of an electrochemicallyactive material—meaning that it is able to chemically react with lithiumions to store and release them reversibly in a controlled manner—mixedif necessary with an electrically conductive additive (such as carbon)and a polymeric binder. A slurry of these components is coated as a thinfilm on a current collector (typically a thin foil of copper oraluminium), thus forming the electrode upon drying.

In the known Li-ion battery technology, the safety limitations ofgraphite anodes upon battery charging is a serious impediment to itsapplication in high-power electronics, automotive and industry. Among awide range of potential alternatives proposed recently, lithium titanate(LTO) and mixed niobium oxides are the main contenders to replacegraphite as the active material of choice for high power, fast-chargeapplications.

Batteries relying on a graphitic anode are fundamentally limited interms of charging rate. Under nominal conditions, lithium ions areinserted into the anode active material upon charging. When chargingrate increases, typical graphite voltage profiles are such that there isa high risk that overpotentials lead to the potential of sites on theanode to become <0 V vs. Li/Li+, which leads to a phenomenon calledlithium dendrite electroplating, whereby lithium ions instead deposit atthe surface of the graphite electrode as lithium metal. This leads toirreversible loss of active lithium and hence rapid capacity fade of thecell. In some cases, these dendritic deposits can grow to such largesizes that they pierce the battery separator and lead to a short-circuitof the cell. This can trigger a catastrophic failure of the cell leadingto a fire or an explosion. Accordingly, the fastest-charging batterieshaving graphitic anodes are limited to charging rates of 5-7 C, butoften much less.

Lithium titanate (LTO) anodes do not suffer from dendrite electroplatingat high charging rate thanks to their high potential (1.6 V vs. Li/Li+),and have excellent cycle life as they do not suffer from significantvolume expansion of the active material upon intercalation of Li ionsdue to their accommodating 3D crystal structure. LTO cells are typicallyregarded as high safety cells for these two reasons. However, LTO is arelatively poor electronic and ionic conductor, which leads to limitedcapacity retention at high rate and resultant power performance, unlessthe material is nanosized to increase specific surface area, andcarbon-coated to increase electronic conductivity. This particle-levelmaterial engineering increases the porosity and specific surface area ofthe active material, and results in a significantly lower achievablepacking density in an electrode. This is significant because it leads tolow density electrodes and a higher fraction of electrochemicallyinactive material (e.g. binder, carbon additive), resulting in muchlower gravimetric and volumetric energy densities.

A key measure of anode performance is the electrode volumetric capacity(mAh/cm³), that is, the amount of electric charges (that is lithiumions) that can be stored per unit volume of the anode. This is animportant factor to determine the overall battery energy density on avolumetric basis (Wh/L) when combined with the cathode and appropriatecell design parameters. Electrode volumetric capacity can beapproximated as the product of electrode density (g/cm³), activematerial specific capacity (mAh/g), and fraction of active material inthe electrode. LTO anodes typically have relatively low specificcapacities (c. 165 mAh/g, to be compared with c. 330 mAh/g for graphite)which, combined with their low electrode densities (typically <2.0g/cm³) and low active material fractions (<90%) discussed above, lead tovery low volumetric capacities (<300 mAh/cm³) and therefore low batteryenergy density and high $/kWh cost in various applications. As a result,LTO batteries/cells are generally limited to specific nicheapplications, despite their long cycle life, fast-charging capability,and high safety.

Mixed niobium oxide structures have been of recent interest for use inLi-ion cells. Zhu et al., J. Mater. Chem. A, 2019, 7, 25537 and Zhu etal., Chem. Commun., 2020,56, 7321-7324 disclose Zn₂Nb₃₄O₈₇ andCu₂Nb₃₄O₈₇ as possible active electrode materials. These papers rely oncomplex particle-level engineering to purportedly achieve goodproperties, e.g. attempting to control particle porosity and morphology.It is believed that the properties of these materials can be improved.For example, these materials may not have sufficient electronicconductivity enough to allow for efficient charging and discharging inLi-ion cells for commercial use, resulting in excess impedance. Inaddition, improvements can still be made in Li ion capacity, coulombicefficiency, and in tuning the voltage profile of charge and discharge.Making these improvements as described herein without the need forextensive nanoscale or particle-level engineering, and without coatings,is an important step to low-cost battery materials for mass marketuptakes. If these improvements are not addressed, then there is excesselectrical resistance in a resultant device and lower energy densities,leading to increased polarisation, reduced power densities, lower energyefficiencies, and increased cost. Accordingly, there remains a need toimprove the properties of Zn₂Nb₃₄O₈₇ and Cu₂Nb₃₄O₈₇ for use inlithium-ion batteries.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides an active electrode materialcomprising a mixed niobium oxide, wherein the mixed niobium oxide hasthe composition M1_(a)M2_(2-a)M3_(b)Nb_(34-b)O_(87-c-d)Q_(d), wherein:

M1 and M2 are different;

M1 is selected from Mg, Ca, Sr, Y, La, Ce, Ti, Zr, Hf, V, Nb, Ta, Cr,Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Ga, In, Si, Ge, Sn, Pb, P, Sb,Bi and mixtures thereof;

M2 is Zn or Cu;

M3 is selected from Mg, Ca, Sr, Y, La, Ce, Ti, Zr, Hf, V, Ta, Cr, Mo, W,Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Ga, In, Si, Ge, Sn, Pb, P, Sb, Bi,and mixtures thereof;

Q is selected from F, Cl, Br, I, N, S, Se, and mixtures thereof;

0≤a<1.0; 0≤b≤3.4; −0.55≤c≤4.35; 0≤d≤4.35;

one or more of a, b, c, and d does not equal 0; and

when a, b, and d equal zero, c is greater than zero.

It will be understood that the composition of the mixed niobium oxidedoes not correspond to stoichiometric Zn₂Nb₃₄O₈₇ or Cu₂Nb₃₄O₈₇. Thepresent inventors have found that by modifying Zn₂Nb₃₄O₈₇ or Cu₂Nb₃₄O₈₇by either incorporating further cations (M1 and/or M3), and/or bycreating an induced oxygen deficiency or excess, and/or by forming mixedanion materials (comprising O and Q), the resulting material hasimproved electrochemical properties, and in particular improvedelectrochemical properties when used as an anode material. When a>0, themixed niobium oxide is modified by partial substitution of M2 (Zn or Cu)by M1. When b>0 the mixed niobium oxide is modified by partialsubstitution of Nb by M3. When c≠0, the mixed niobium oxide is modifiedby oxygen deficiency or excess. When d>0 the mixed niobium oxide ismodified by partial substitution of O by Q. The inventors have foundthat materials according to the invention have improved electronicconductivity, and improved coulombic efficiency, and improvedde-lithiation voltage at high C-rates, compared to unmodified ‘base’Zn₂Nb₃₄O₈₇, as shown by the present examples. These are importantresults in demonstrating the advantages of the material of the inventionfor use in high-power batteries designed for fast charge/discharge.

The active electrode material of the invention is particularly useful inelectrodes, preferably for use in anodes for lithium-ion or sodium-ionbatteries. Therefore, in a further implementation of the invention theactive electrode material of the first aspect comprises the mixedniobium oxide and at least one other component; optionally wherein theat least one other component is selected from a binder, a solvent, aconductive additive, a different active electrode material, and mixturesthereof. Such a composition is useful for fabricating an electrode. Afurther implementation of the invention is an electrode comprising theactive electrode material of the first aspect in electrical contact witha current collector. A further implementation of the invention is anelectrochemical device comprising an anode, a cathode, and anelectrolyte disposed between the anode and the cathode, wherein theanode comprises an active electrode material according to the firstaspect; optionally wherein the electrochemical device is a lithium-ionbattery or a sodium-ion battery.

In a second aspect, the invention provides a method of making a mixedniobium oxide as defined by the first aspect, the method comprisingsteps of: providing one or more precursor materials; mixing saidprecursor materials to form a precursor material mixture; and heattreating the precursor material mixture in a temperature range from 400°C.-1350° C. or 800-1350° C., thereby providing the mixed niobium oxide.This represents a convenient and efficient method of making the activeelectrode material of the first aspect.

The invention includes the combination of the aspects and featuresdescribed herein except where such a combination is clearlyimpermissible or expressly avoided.

SUMMARY OF THE FIGURES

The principles of the invention will now be discussed with reference tothe accompanying figures.

FIG. 1 : Powder XRD of Samples 1-4

FIG. 2 : Powder XRD of Samples 5-12.

DETAILED DESCRIPTION OF THE INVENTION

Aspects and embodiments of the present invention will now be discussedwith reference to the accompanying figures. Further aspects andembodiments will be apparent to those skilled in the art. All documentsmentioned in this text are incorporated herein by reference.

The term “mixed niobium oxide” (MNO) may refer to an oxide comprisingniobium and at least one other cation. MNO materials have a high redoxvoltage vs. Lithium >0.8V, enabling safe and long lifetime operation,crucial for fast charging battery cells. Moreover, niobium cations canhave two redox reactions per atom, resulting in higher theoreticalcapacities than, for example, LTO. The mixed niobium oxide describedherein is derived from the base structure of Zn₂Nb₃₄O₈₇ or Cu₂Nb₃₄O₈₇.

Zn₂Nb₃₄O₈₇ or Cu₂Nb₃₄O₈₇ may be considered to have a ReO₃-derivedMO_(3-x) crystal structure. Preferably, the mixed niobium oxide has aWadsley-Roth crystal structure. Wadsley-Roth crystal structures areconsidered to be a crystallographic off-stoichiometry of the MO₃ (ReO₃)crystal structure containing crystallographic shear, with simplifiedformula of MO_(3-x). As a result, these structures typically contain[MO₆] octahedral subunits in their crystal structure. The MNO materialswith these structures are believed to have advantageous properties foruse as active electrode materials, e.g. in lithium-ion batteries.

The open tunnel-like MO₃ crystal structure of MNO materials also makesthem ideal candidates for having high capacity for Li ion storage andhigh rate intercalation/de-intercalation. The crystallographicoff-stoichiometry present in the MNO structure causes the Wadsley-Rothcrystallographic superstructure.

These superstructures, compounded by other qualities such as theJahn-Teller effect and enhanced crystallographic disorder by making useof multiple mixed cations, stabilise the crystal and keep the tunnelsopen and stable during intercalation, enabling extremely high rateperformance due to high Li-ion diffusion rates (reported as ˜10⁻¹³ cm²s⁻¹).

The crystal formulae of Zn₂Nb₃₄O₈₇ or Cu₂Nb₃₄O₈₇ can be described ashaving a 3×4×∞ crystallographic block structure composed of [MO₆]octahedra, where M is Cu, Zn, or Nb. The Cu and Zn octahedra may berandomly distributed in the structure or may have a preference forparticular sites such as at the edge, or corner of the blocks. Thisequates to ⅔ of one Zn or Cu cation per block. The crystal formulae ofZn₂Nb₃₄O₈₇ can be described as an isostructural phase to Cu₂Nb₃₄O₈₇ withslight differences in some bond lengths and bond enthalpies.

The total crystal composition of the materials described herein arepreferably charge neutral and thermodynamically favourable to follow theabove description. Oxygen-deficient structures e.g. through introductionof oxygen vacancy point defects are preferable when reducing thematerial's electrical resistance such that M_(x)O_(y) becomesM_(x)O_(y-σ). Oxygen deficient structures may contain shear defects.

Structures that have had cations (i.e. Zn, Cu, and Nb) or anions (i.e.O) substituted may have been so with matching valency (i.e. a 5+ cationfor equal proportions of a 4+ and 6+ cation) or with unmatched valency,which can induce oxygen deficiency or excess if substitution takes placeat equivalent crystal sites. Substitution may also take place atdifferent crystal sites, such as interstitial sites.

The crystal structure of a material may be determined by analysis ofX-ray diffraction (XRD) patterns, as is widely known. For instance, XRDpatterns obtained from a given material can be compared to known XRDpatterns to confirm the crystal structure, e.g. via public databasessuch as the ICDD crystallography database. Rietveld analysis can also beused to determine the crystal structure of materials, in particular forthe unit cell parameters. Therefore, the active electrode material mayhave a Wadsley-Roth crystal structure, as determined by X-raydiffraction.

Preferably, the crystal structure of the mixed niobium oxide, asdetermined by X-ray diffraction, corresponds to the crystal structure ofZn₂Nb₃₄O₈₇ or Cu₂Nb₃₄O₈₇; most preferably Zn₂Nb₃₄O₈₇. In this way, itcan be confirmed that the ‘base’ material has been modified withoutsignificantly affecting the crystal structure, which is believed to haveadvantageous properties for use as an active electrode material. Thecrystal structure of Zn₂Nb₃₄O₈₇ may be found at ICDD crystallographydatabase entry JCPDS 28-1478.

The mixed niobium oxide with cation/anion exchange may have unit cellparameters a, b, and c wherein a is 15.52-15.58 Å preferably 15.53-15.57Å, b is 3.79-3.84 Å preferably 3.80-3.83 Å, and c=20.53-20.66 Åpreferably 20.54-20.65 Å. The mixed niobium oxide may have unit cellparameters α and γ each being about 90°, preferably wherein α=γ=90°;whereas β is 113.05-113.75⁰ preferably 113.08-113.69° and unit cellvolume is 1115-1135 Å³ preferably 1117-1133 Å³. Unit cell parameters maybe determined by X-ray diffraction. The mixed niobium oxide may have acrystallite size of 5-150 nm, preferably 30-60 nm, determined accordingto the Scherrer equation.

Here the term ‘corresponds’ is intended to reflect that peaks in anX-ray diffraction pattern may be shifted by no more than 0.5 degrees(preferably shifted by no more than 0.25 degrees, more preferablyshifted by no more than 0.1 degrees) from corresponding peaks in anX-ray diffraction pattern of the material listed above.

The mixed niobium oxide has the compositionM1_(a)M2_(2-a)M3_(b)Nb_(34-b)O_(87-c-d)Q_(d), wherein:

M1 and M2 are different;

M1 is selected from Mg, Ca, Sr, Y, La, Ce, Ti, Zr, Hf, V, Nb, Ta, Cr,Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Ga, In, Si, Ge, Sn, Pb, P, Sb,Bi and mixtures thereof;

M2 is Zn or Cu;

M3 is selected from Mg, Ca, Sr, Y, La, Ce, Ti, Zr, Hf, V, Ta, Cr, Mo, W,Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Ga, In, Si, Ge, Sn, Pb, P, Sb, Bi,and mixtures thereof;

Q is selected from F, Cl, Br, I, N, S, Se, and mixtures thereof;

0≤a<1.0; 0≤b≤3.4; −0.55≤c≤4.35; 0≤d≤4.35;

one or more of a, b, c, and d does not equal 0; and

when a, b, and d equal zero, c is greater than zero.

By ‘and mixtures thereof’, it is intended that M1, M3, and Q may eachrepresent two or more elements from their respective lists. An exampleof such a material is Mg_(0.1)Ge_(0.1)Zn_(1.8)Nb₃₄O_(87.1). Here, M1 isMg_(a′)Ge_(a″) (where a′+a″=a), M2 is Zn, a=0.2, b=0, c=−0.1, d=0. Here,c has been calculated assuming that each cation adopts its typicaloxidation state, i.e. Mg²⁺, Ge⁴⁺, Zn²⁺, and Nb⁵⁺.

The precise values of a, b, c, d within the ranges defined may beselected to provide a charge balanced, or substantially charge balanced,crystal structure. Additionally or alternatively, the precise values ofa, b, c, d within the ranges defined may be selected to provide athermodynamically stable, or thermodynamically metastable, crystalstructure.

When exchange of the cations or anions in the structure (i.e. Zn, Cu,Nb, O) have taken place without preserving the initial valency, this cangive rise to both oxygen deficiency and excess. For example, a materialthat substitutes Zn²⁺ by Ge⁴⁺ to some extent will demonstrate minoroxygen excess (i.e. ZnO vs GeO₂), whereas substitution of Nb⁵⁺ by Al³⁺will show a minor oxygen deficiency (i.e. Nb₂O₅ vs Al₂O₃).

Oxygen deficiency can also be induced through thermal treatment in inertor reducing conditions, which results in induced oxygen vacancy defectsin the structure.

There may be partial oxidation or partial reduction to compensate forexchange which does not preserve the initial valency. For example,substitution of Zn²⁺ by Ge⁴⁺ may be compensated at least in part byreduction of some Nb⁵⁺ to Nb⁴⁺.

M2 is Zn or Cu. Preferably, M2 is Zn in which case the material is basedon Zn₂Nb₃₄O₈₇.

M1 is a cation which substitutes for M2 in the crystal structure. M1 maybe selected from Mg, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni,Cu, Zn, Cd, B, Al, Ga, Si, Ge, Sn, P, and mixtures thereof; preferablyMg, Ti, Zr, V, Nb, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Si, Ge, P,and mixtures thereof; most preferably Mg, Zr, V, Cr, Mo, W, Fe, Cu, Zn,Al, Ge, P, and mixtures thereof. M1 may have a different valency thanM2²⁺. This gives rise to oxygen deficiency or excess. Optionally, M1 hasan equal or higher valency than M2²⁺, preferably higher.

M1 may also be selected from each of the specific elements used as suchin the examples.

When more than one element is present as M1 or M3 it will be understoodthat the valency refers to M1 or M3 as a whole. For example, if 25 at %of M1 is Ti and 75 at % of M1 is W the valency M1 is 0.25×4 (thecontribution from Ti)+0.75×6 (the contribution from W).

M1 preferably has a different ionic radius than M2²⁺, most preferably asmaller ionic radius. This gives rise to changing unit cell size andlocal distortions in crystal structure, providing the advantagesdiscussed herein. Ionic radii referred to herein are the Shannon ionicradii (available at R. D. Shannon, Acta Cryst., A32, 1976, 751-767) atthe coordination and valency that the ion would be expected to adopt inthe crystal structure of the mixed niobium oxide. For example, thecrystal structure of Zn₂Nb₃₄O₈₇ includes Nb⁵⁺O₆ octahedra and Zn²⁺O₆octahedra. Accordingly, when M3 is Zr the ionic radius is taken as thatof 6-coordinate Zr⁴⁺ since this is typical valency and coordination ofZr when replacing Nb in Zn₂Nb₃₄O₈₇.

The amount of M1 is defined by a, meeting the criterion 0≤a<1.0. a maybe 0≤a≤0.6, preferably 0≤a≤0.2. Most preferably, a>0, for examplea≥0.01. Higher values of a may be more readily achieved when M1 has thesame valency as M2. When M1 comprises a cation with a 2+ valency (forexample Mg) a may be 0≤a<1.0. When M1 does not comprise a cation with a2+ valency a may be 0≤a≤0.15.

M3 is a cation which substitutes for Nb in the crystal structure. M3 maybe selected from Mg, Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu,Zn, Cd, B, Al, Ga, Si, Sn, P, and mixtures thereof; preferably Mg, Ti,Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, Cd, B, Al, Si, P, and mixturesthereof; most preferably Ti, Zr, V, Cr, Mo, W, Fe, Cu, Zn, Al, P, andmixtures thereof. M3 may have a different valency than Nb⁵⁺. This givesrise to oxygen deficiency or excess. Preferably, M3 has a lower valencythan Nb⁵⁺. This gives rise to oxygen deficiency, i.e. the presence ofoxygen vacancies providing the advantages discussed herein.

M3 may also be selected from each of the specific elements used as suchin the examples.

M3 preferably has a different ionic radius than Nb⁵⁺, most preferably alarger ionic radius. This gives rise to changing unit cell size andlocal distortions in crystal structure, providing the advantagesdiscussed herein.

The amount of M3 is defined by b, meeting the criterion 0≤b≤3.4. b maybe 0≤b≤1.5, preferably 0≤b≤0.3. In each of these cases b may be >0, e.g.b≥0.01. Higher values of b may be more readily achieved when M3 has thesame valency as Nb⁵⁺. When M3 comprises a cation with a 5+ valency (forexample Ta) b may be 0≤b≤3.4. When M3 does not comprise a cation with a5+ valency b may be 0≤b≤0.2.

Surprisingly, it has been found that the cation-substitution approach inaccordance with the invention can lead to a mixed niobium oxide that ismore economical to synthesise than the unmodified ‘base’ material.Optionally, both a and b are >0. When both a and b are >0 the ‘base’material has been substituted at both the M2 site and at the Nb site.

c reflects the oxygen content of the mixed niobium oxide. When c isgreater than 0, it forms an oxygen-deficient material, i.e. the materialhas oxygen vacancies. Such a material would not have precise chargebalance without changes to cation oxygen state, but is considered to be“substantially charge balanced” as indicated above. Alternatively, c mayequal 0, in which it is not an oxygen-deficient material. c may be below0, which is a material with oxygen-excess. c may be −0.25≤c≤4.35.

When c is 4.35, the number of oxygen vacancies is equivalent to 5% ofthe total oxygen in the crystal structure. c may be greater than 0.0435,greater than 0.087, greater than 0.174, or greater than 0.435. c may bebetween 0 and 2, between 0 and 0.75, between 0 and 0.5, or between 0 and0.25. For example, c may satisfy 0.01≤c≤4.35. When the material isoxygen-deficient, for example with induced oxygen deficiency, theelectrochemical properties of the material may be improved, for example,resistance measurements may show improved conductivity in comparison toequivalent non-oxygen-deficient materials. As will be understood, thepercentage values expressed herein are in atomic percent.

The invention relates to mixed niobium oxides which may comprise oxygenvacancies (oxygen-deficient mixed niobium oxides), or which may haveoxygen excess. Oxygen vacancies may be formed in a mixed niobium oxideby the sub-valent substitution of a base material as described above,and oxygen excess may be formed in a mixed niobium oxide by substitutionfor increased valency. Oxygen vacancies may also be formed by heating amixed niobium oxide under reducing conditions, which may be termedforming induced oxygen deficiency. The amount of oxygen vacancies andexcess may be expressed relative to the total amount of oxygen in thebase material, i.e. the amount of oxygen in the un-substituted material(e.g. Zn₂Nb₃₄O₈₇).

A number of methods exist for determining whether oxygen deficiency,e.g. oxygen vacancies, is present in a material. For example,Thermogravimetric Analysis (TGA) may be performed to measure the masschange of a material when heated in air atmosphere. A materialcomprising oxygen vacancies can increase in mass when heated in air dueto the material “re-oxidising” and the oxygen vacancies being filled byoxide anions. The magnitude of the mass increase may be used to quantifythe concentration of oxygen vacancies in the material, on the assumptionthat the mass increase occurs entirely due to the oxygen vacancies beingfilled. It should be noted that a material comprising oxygen vacanciesmay show an initial mass increase as the oxygen vacancies are filled,followed by a mass decrease at higher temperatures if the materialundergoes thermal decomposition. Moreover, there may be overlapping massloss and mass gain processes, meaning that some materials comprisingoxygen vacancies may not show a mass gain (and sometimes not a mass lossor gain) during TGA analysis.

Other methods of determining whether oxygen deficiency e.g. oxygenvacancies, is present include Raman spectroscopy, electron paramagneticresonance (EPR), X-ray photoelectron spectroscopy (XPS, e.g. of oxygen 1s and/or and of cations in a mixed oxide), X-ray absorption near-edgestructure (XANES, e.g. of cations in a mixed metal oxide), and TEM (e.g.scanning TEM (STEM) equipped with high-angle annular darkfield (HAADF)and annular bright-field (ABF) detectors). The presence of oxygendeficiency can be qualitatively determined by assessing the colour of amaterial relative to a non-oxygen-deficient sample of the same material,indicative of changes to its electronic band structure throughinteraction with light. For example, non-oxygen deficient stoichiometricZn₂Nb₃₄O₈₇ has a white colour. Zn₂Nb₃₄O_(<87) with induced oxygendeficiency has a grey/black. The presence of vacancies can also beinferred from the properties, e.g. electrical conductivity, of astoichiometric material compared to those of an oxygen-deficientmaterial.

When d>0, additional anions Q are introduced into the mixed niobiumoxide. Due to their differing electronic structure (i.e. F⁻ vs O²⁻), anddiffering ionic radii (6-coordinate O²⁻=1.40 Å, 6-coordinate F⁻=1.33 Å)they may improve electrochemical performance in the active material.This is due to altering unit cell characteristics with differing ionicradii allowing for improved Li ion capacity, or improved Coulombicefficiencies by improving reversibility. They may additionally improveelectrical conductivity as for oxygen vacancy defects, or sub-valentcation substitutions, by altering the electronic structure of thecrystal (i.e. doping effects). d may be 0≤d≤3.0, or 0≤d≤2.17. In each ofthese cases d may be >0. Q may be selected from F, Cl, N, S, andmixtures thereof; or F, N, and mixtures thereof; or Q is F.

Optionally d=0, in which case the material has the compositionM1_(a)M2_(2-a)M3_(b)Nb_(34-b)O_(87-c) where M1, M2, M3, a, b, and c areas defined herein. Advantageously, materials where d=0 are free fromanion Q and may be easier to synthesise.

When a>0 and b=d=0 the material has the compositionM1_(a)M2_(2-a)Nb₃₄O_(87-c) where M1, M2, a, and c are as defined herein,for example 0≤c≤4.35. This represents a material which has been modifiedat the M2 site and optionally modified by induced oxygen deficiency.Such materials represent a particularly effective way to improve theproperties of the ‘base’ oxide M2₂Nb₃₄O₈₇ by simple synthetic means.Here, M1 may represent Ti, Mg, V, Cr, W, Zr, Mo, Cu, Ga, Ge, Ni, Al, Hf,Ta, Zn and mixtures thereof; preferably Ti, Mg, V, Cr, W, Zr, Mo, Ga,Ge, Al, Zn, and mixtures thereof.

When a=b=d=0 and c>0 the material has the composition M2₂Nb₃₄O_(87-c)where M2 and c are as defined herein. This represents a material whichhas been modified solely by inducing oxygen deficiency, providingimproved properties as shown in the examples. For example, materialswhere a=b=d=0 and c>0 have been found to have surprisingly improvedelectronic conductivity.

It will be understood that the discussion of the variables of thecomposition (M1, M2, M3, Q, a, b, c, and d) is intended to be read incombination. For example, preferably M1 is selected from Mg, Ti, Zr, V,Nb, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, B, Al, Si, Ge, P, and mixturesthereof and M3 is selected from Mg, Ti, Zr, V, Cr, Mo, W, Mn, Fe, Co,Ni, Cu, Zn, Cd, B, Al, Si, P, and mixtures thereof and Q is selectedfrom F, Cl, N, S, and mixtures thereof. Preferably 0≤a≤0.6, 0≤b≤1.5,0≤c≤4.35, and 0≤d≤3.0.

For example, the mixed niobium oxide may have the compositionM1_(a)M2_(2-a)M3_(b)Nb_(34-b)O_(87-c-d)Q_(d), wherein:

M1 and M2 are different;

M1 is selected from Mg, Ti, Zr, V, Nb, Cr, Mo, W, Mn, Fe, Co, Ni, Cu,Zn, B, Al, Si, Ge, P, and mixtures thereof;

M2 is Zn or Cu;

M3 is selected from Mg, Ti, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn,Cd, B, Al, Si, P, and mixtures thereof;

Q is selected from F, N, and mixtures thereof; 0≤a≤0.6; 0≤b≤1.5;−0.55≤c≤4.35; 0≤d≤4.35;

one or more of a, b, c, and d does not equal 0; and

when a, b, and d equal zero, c is greater than zero.

For example, the mixed niobium oxide may have the compositionM1_(a)Zn_(2-a)M3_(b)Nb_(34-b)O_(87-c-d)Q_(d), wherein:

M1 is selected from Mg, Zr, V, Cr, Mo, W, Fe, Cu, Al, Ge, P, andmixtures thereof;

M3 is selected from Ti, Zr, V, Cr, Mo, W, Fe, Cu, Zn, Al, P, andmixtures thereof;

Q is selected from F, N, and mixtures thereof;

0<a≤0.2; 0≤b≤0.3; 0≤c≤4.35; 0≤d≤3.0.

M1, M3, and Q may also be selected from each of the specific elementsused as these dopants in the examples.

Optionally, the mixed niobium oxide is free from titanium.

The mixed niobium oxide may have the compositionM1_(a)M2_(2-a)M3_(b)Nb_(34-b)O_(87-c), wherein:

M1 is selected from Cr, Al, Ge, and mixtures thereof, preferably whereinM1 is Cr;

M2 is Zn or Cu, preferably wherein M2 is Zn;

M3 is selected from Ti, Zr, Fe, and mixtures thereof and optionallycomprises Ti, preferably wherein M3 is selected from Ti, Zr, andmixtures thereof and optionally comprises Ti, most preferably wherein M3is Ti;

0<a<1.0, preferably 0.01<a<1.0;

0<b≤1.5, preferably 0.01<b<1.0;

−0.5≤c≤4.35, preferably −0.5≤c≤2, most preferably c=0.

The mixed niobium oxide may have the compositionCr_(a)Zn_(2-a)M3_(b)Nb_(34-b)O_(87-c), wherein:

M3 is selected from Ti, Zr, and mixtures thereof and optionallycomprises Ti, preferably wherein M3 is Ti;

0.01<a<1.0, preferably 0.1<a<1.0;

0.01<b<1.0, preferably 0.1<b<1.0;

−0.5≤c≤2, preferably c=0.

The mixed niobium oxide may further comprise Li and/or Na. For example,Li and/or Na may enter the crystal structure when the mixed niobiumoxide is used in a metal-ion battery electrode.

The mixed niobium oxide is preferably in particulate form. The materialmay have a D₅₀ particle diameter in the range of 0.1-100 μm, or 0.5-50μm, or 1-20 μm. These particle sizes are advantageous because they areeasy to process and fabricate into electrodes. Moreover, these particlesizes avoid the need to use complex and/or expensive methods forproviding nanosized particles. Nanosized particles (e.g. particleshaving a D₅₀ particle diameter of 100 nm or less) are typically morecomplex to synthesise and require additional safety considerations.

The mixed niobium oxide may have a D₁₀ particle diameter of at least0.05 μm, or at least 0.1 μm, or at least 0.5 μm, or at least 1 μm. Bymaintaining a D₁₀ particle diameter within these ranges, the potentialfor parasitic reactions in a Li ion cell is reduced from having reducedsurface area, and it is easier to process with less binder in theelectrode slurry.

The mixed niobium oxide may have a D₉₀ particle diameter of no more than200 μm, no more than 100 μm, no more than 50 μm, or no more than 20 μm.By maintaining a Do particle diameter within these ranges, theproportion of the particle size distribution with large particle sizesis minimised, making the material easier to manufacture into ahomogenous electrode.

The term “particle diameter” refers to the equivalent spherical diameter(esd), i.e. the diameter of a sphere having the same volume as a givenparticle, where the particle volume is understood to include the volumeof any intra-particle pores. The terms “D_(n)” and “D_(n) particlediameter” refer to the diameter below which n % by volume of theparticle population is found, i.e. the terms “D₅₀” and “D₅₀ particlediameter” refer to the volume-based median particle diameter below which50% by volume of the particle population is found. Where a materialcomprises primary crystallites agglomerated into secondary particles, itwill be understood that the particle diameter refers to the diameter ofthe secondary particles. Particle diameters can be determined by laserdiffraction. Particle diameters can be determined in accordance with ISO13320:2009, for example using Mie theory.

The mixed niobium oxide may have a BET surface area in the range of0.1-100 m²/g, or 0.5-50 m²/g, or 1-20 m²/g. In general, a low BETsurface area is preferred in order to minimise the reaction of the mixedniobium oxide with the electrolyte, e.g. minimising the formation ofsolid electrolyte interphase (SEI) layers during the firstcharge-discharge cycle of an electrode comprising the material. However,a BET surface area which is too low results in unacceptably low chargingrate and capacity due to the inaccessibility of the bulk of the mixedniobium oxide to metal ions in the surrounding electrolyte.

The term “BET surface area” refers to the surface area per unit masscalculated from a measurement of the physical adsorption of gasmolecules on a solid surface, using the Brunauer-Emmett-Teller theory.

For example, BET surface areas can be determined in accordance with ISO9277:2010.

The specific capacity/reversible delithiation capacity of the mixedniobium oxide may be 180 mAh/g or more, 190 mAh/g or more, or 197 mAh/gor more. Here, specific capacity is defined as that measured in the 2ndcycle of a half cell galvanostatic cycling test at a rate of 0.1C with avoltage window of 1.1-3.0V vs Li/Li+ in a half cell. It may beadvantageous to provide materials having a high specific capacity, asthis can provide improved performance in an electrochemical devicecomprising the mixed niobium oxide.

When formulated or coated as an electrode according to the belowdescription (optionally with conductive carbon additive and bindermaterials), the sheet resistance of the active electrode material may be2.5 kΩ per square or less, more preferably 1.2 kΩ per square or less,which may be measured as defined in the examples. Sheet resistance canbe a useful proxy measurement of the electronic conductivity of suchmaterials. It may be advantageous to provide materials having a suitablylow sheet resistance, as this can provide improved performance in anelectrochemical device comprising the mixed niobium oxide.

The mixed niobium oxide may have a lithium diffusion rate greater than10⁻¹⁵ cm² s⁻¹, or more preferably greater than 10⁻¹³ cm² s⁻¹. It may beadvantageous to provide materials having a suitably high lithiumdiffusion rate, as this can provide improved performance in anelectrochemical device comprising the mixed niobium oxide.

The mixed niobium oxide may be able to form composite electrodes with asuitable binder and conductive additive according to the belowdescription to provide an electrode density of 2.5 g/cm³ or more aftercalendaring. This enables a composite electrode with an electrodeporosity (calculated by the measured electrode density/average of thetrue density of each component) in the range of 30-40%, in-line withindustrial requirements for high energy and high power cells. Forexample, electrode densities of up to 2.9 g/cm³ have been achieved. Itmay be advantageous to provide materials having such an electrodedensity, as this can provide improved performance in an electrochemicaldevice comprising the mixed niobium oxide. Specifically, when theelectrode density is high, high volumetric capacities can be achieved,as gravimetric capacity×electrode density×mixed niobium oxidefraction=volumetric capacity.

Initial coulombic efficiency has been measured as the difference in thelithiation and de-lithiation capacity on the 1^(st) charge/dischargecycle at C/10 in a half-cell. The initial coulombic efficiency of theactive electrode material may be greater than 97.9%, or greater than98.8%. It may be advantageous to provide materials having a suitablyhigh initial coulombic efficiency, as this can provide improvedperformance in an electrochemical device comprising the mixed niobiumoxide.

The active electrode material of the first aspect of the invention maycomprise the mixed niobium oxide and at least one other component,optionally wherein the at least one other component is selected from abinder, a solvent, a conductive additive, a different active electrodematerial, and mixtures thereof. Such a composition is useful forpreparing an electrode, e.g. an anode for a lithium-ion battery.Preferably, the different active electrode material is selected from adifferent mixed niobium oxide having a composition as defined by thefirst aspect, a lithium titanium oxide, a niobium oxide, and mixturesthereof.

Alternatively, the active electrode material may consist of the mixedniobium oxide.

The active electrode material may comprise the mixed niobium oxide and alithium titanium oxide, preferably a mixture of the mixed niobium oxideand a lithium titanium oxide.

The lithium titanium oxide preferably has a spinel or ramsdellitecrystal structure, e.g. as determined by X-ray diffraction. An exampleof a lithium titanium oxide having a spinel crystal structure isLi₄Ti₅O₁₂. An example of a lithium titanium oxide having a ramsdellitecrystal structure is Li₂Ti₃O₇. These materials have been shown to havegood properties for use as active electrode materials. Therefore, thelithium titanium oxide may have a crystal structure as determined byX-ray diffraction corresponding to Li₄Ti₅O₁₂ and/or Li₂Ti₃O₇. Thelithium titanium oxide may be selected from Li₄Ti₅O₁₂, Li₂Ti₃O₇, andmixtures thereof.

The lithium titanium oxide may be doped with additional cations oranions. The lithium titanium oxide may be oxygen deficient. The lithiumtitanium oxide may comprise a coating, optionally wherein the coating isselected from carbon, polymers, metals, metal oxides, metalloids,phosphates, and fluorides.

The lithium titanium oxide may be synthesised by conventional ceramictechniques, for example solid-state synthesis or sol-gel synthesis.Alternatively, the lithium titanium oxide may be obtained from acommercial supplier.

The lithium titanium oxide is in preferably in particulate form. Thelithium titanium oxide may have a D₅₀ particle diameter in the range of0.1-50 μm, or 0.25-20 μm, or 0.5-15 μm. The lithium titanium oxide mayhave a D₁₀ particle diameter of at least 0.01 μm, or at least 0.1 μm, orat least 0.5 μm. The lithium titanium oxide may have a D₁₀ particlediameter of no more than 100 μm, no more than 50 μm, or no more than 25μm. By maintaining a D₉₀ particle diameter in this range the packing oflithium titanium oxide particles in the mixture with mixed niobium oxideparticles is improved.

Lithium titanium oxides are typically used in battery anodes at smallparticle sizes due to the low electronic conductivity of the material.In contrast, the mixed niobium oxide as defined herein may be used atlarger particle sizes since it typically has a higher lithium iondiffusion coefficient than lithium titanium oxide. Advantageously, inthe composition the lithium titanium oxide may have a smaller particlesize than the mixed niobium oxide, for example such that the ratio ofthe D₅₀ particle diameter of the lithium titanium oxide to the D₅₀particle diameter of the mixed niobium oxide is in the range of 0.01:1to 0.9:1, or 0.1:1 to 0.7:1. In this way, the smaller lithium titaniumoxide particles may be accommodated in the voids between the largermixed niobium oxide particles, increasing the packing efficiency of thecomposition.

The lithium titanium oxide may have a BET surface area in the range of0.1-100 m²/g, or 1-50 m²/g, or 3-30 m²/g.

The ratio by mass of the lithium titanium oxide to the mixed niobiumoxide may be in the range of 0.5:99.5 to 99.5:0.5, preferably in therange of 2:98 to 98:2. In one implementation the active electrodematerial comprises a higher proportion of the lithium titanium oxidethan the mixed niobium oxide, e.g. the ratio by mass of at least 2:1, atleast 5:1, or at least 8:1. Advantageously, this allows the mixedniobium oxide to be incrementally introduced into existing electrodesbased on lithium titanium oxides without requiring a large change inmanufacturing techniques, providing an efficient way of improving theproperties of existing electrodes. In another implementation the activeelectrode material has a higher proportion of the mixed niobium oxidethan the lithium titanium oxide, e.g. such that the ratio by mass of thelithium titanium oxide to the mixed niobium oxide is less than 1:2, orless than 1:5, or less than 1:8.

Advantageously, this allows for the cost of the active electrodematerial to be reduced by replacing some of the mixed niobium oxide withlithium titanium oxide.

The active electrode material may comprise the mixed niobium oxide and aniobium oxide. The niobium oxide may be selected from Nb₁₂O₂₉, NbO₂,NbO, and Nb₂O₅. Preferably, the niobium oxide is Nb₂O₅.

The niobium oxide may be doped with additional cations or anions, forexample provided that the crystal structure of the niobium oxidecorresponds to the crystal structure of an oxide consisting of Nb and O,e.g. Nb₁₂O₂₉, NbO₂, NbO, and Nb₂O₅. The niobium oxide may be oxygendeficient. The niobium oxide may comprise a coating, optionally whereinthe coating is selected from carbon, polymers, metals, metal oxides,metalloids, phosphates, and fluorides.

The niobium oxide may have the crystal structure of Nb₁₂O₂₉, NbO₂, NbO,or Nb₂O₅ as determined by X-ray diffraction. For example, the niobiumoxide may have the crystal structure of orthorhombic Nb₂O₅ or thecrystal structure of monoclinic Nb₂O₅. Preferably, the niobium oxide hasthe crystal structure of monoclinic Nb₂O₅, most preferably the crystalstructure of H—Nb₂O₅. Further information on crystal structures of Nb₂O₅may be found at Griffith et al., J. Am. Chem. Soc. 2016, 138, 28,8888-8899.

The niobium oxide may be synthesised by conventional ceramic techniques,for example solid-state synthesis or sol-gel synthesis. Alternatively,the niobium oxide may be obtained from a commercial supplier.

The niobium oxide is in preferably in particulate form. The niobiumoxide may have a D₅₀ particle diameter in the range of 0.1-100 μm, or0.5-50 μm, or 1-20 μm. The niobium oxide may have a D₁₀ particlediameter of at least 0.05 μm, or at least 0.5 μm, or at least 1 μm. Theniobium oxide may have a D₉₀ particle diameter of no more than 100 μm,no more than 50 μm, or no more than 25 μm. By maintaining a D₉₀ particlediameter in this range the packing of niobium oxide particles in themixture with mixed niobium oxide particles is improved.

The niobium oxide may have a BET surface area in the range of 0.1-100m²/g, or 1-50 m²/g, or 1-20 m²/g.

The ratio by mass of the niobium oxide to the mixed niobium oxide may bein the range of 0.5:99.5 to 99.5:0.5, or in the range of 2:98 to 98:2,or preferably in the range of 15:85 to 35:55.

The invention also provides an electrode comprising the active electrodematerial of the first aspect of the invention in electrical contact witha current collector. The electrode may form part of a cell. Theelectrode may form an anode as part of metal-ion battery, optionally alithium-ion battery.

The invention also provides the use of the active electrode material ofthe first aspect of the invention in an anode for a metal-ion battery,optionally wherein the metal-ion battery is a lithium-ion battery.

A further implementation of the invention is an electrochemical devicecomprising an anode, a cathode, and an electrolyte disposed between theanode and the cathode, wherein the anode comprises an active electrodematerial according to the first aspect of the invention; optionallywherein the electrochemical device is metal-ion battery such as alithium-ion battery or a sodium-ion battery. Preferably, theelectrochemical device is a lithium-ion battery having a reversibleanode active material specific capacity of greater than 190 mAh/g at 20mA/g, wherein the battery can be charged and discharged at currentdensities relative to the anode active material of 200 mA/g or more, or1000 mA/g or more, or 2000 mA/g or more, or 4000 mA/g or more whilstretaining greater than 70% of the initial cell capacity at 20 mA/g. Ithas been found that use of the active electrode materials of the firstaspect of the invention can enable the production of a lithium-ionbattery with this combination of properties, representing a lithium-ionbattery that is particularly suitable for use in applications where highcharge and discharge current densities are desired. Notably, theexamples have shown that active electrode materials according to thefirst aspect of the invention have improved electronic conductivity andimproved delithiation voltage at high C-rates.

The mixed niobium oxide may be synthesised by conventional ceramictechniques. For example, the material be made by one or more ofsolid-state synthesis or sol-gel synthesis. The material mayadditionally be synthesised by one or more of alternative techniquescommonly used, such as hydrothermal or microwave hydrothermal synthesis,solvothermal or microwave solvothermal synthesis, coprecipitationsynthesis, spark or microwave plasma synthesis, combustion synthesis,electrospinning, and mechanical alloying.

The second aspect of the invention provides a method of making a mixedniobium oxide as defined by the first aspect, the method comprisingsteps of: providing one or more precursor materials; mixing saidprecursor materials to form a precursor material mixture; and heattreating the precursor material mixture in a temperature range from 400°C.-1350° C. or 800-1350° C., thereby providing the mixed niobium oxide.

To provide a mixed niobium oxide comprising element Q the method mayfurther comprise the steps of: mixing the mixed niobium oxide with aprecursor comprising element Q to provide a further precursor materialmixture; and heat treating the further precursor material mixture in atemperature range from 300-1200° C. or 800-1200° C. optionally underreducing conditions, thereby providing the mixed niobium oxidecomprising element Q.

For example, to provide a mixed niobium oxide comprising N as element Q,the method may further comprise the steps of: mixing the mixed niobiumoxide with a precursor comprising N (for example melamine) to provide afurther precursor material mixture; and heat treating the furtherprecursor material mixture in a temperature range from 300-1200° C.under reducing conditions (for example in N₂), thereby providing themixed niobium oxide comprising N as element Q.

For example, to provide a mixed niobium oxide comprising F as element Q,the method may further comprise the steps of: mixing the mixed niobiumoxide with a precursor comprising F (for example polyvinylidenefluoride) to provide a further precursor material mixture; and heattreating the further precursor material mixture in a temperature rangefrom 300-1200° C. under oxidising conditions (for example in air),thereby providing the mixed niobium oxide comprising F as element Q.

The method may comprise the further step of heat treating the mixedniobium oxide or the mixed niobium oxide comprising element Q in atemperature range from 400-1350° C. or 800-1350° C. under reducingconditions, thereby inducing oxygen vacancies in the mixed niobiumoxide. The induced oxygen vacancies may be in addition to oxygenvacancies already present in the mixed niobium oxide, e.g. alreadypresent due to sub-valent substitution of M2 and/or Nb with M1 and/orM3. Alternatively, the induced oxygen vacancies may be new oxygenvacancies, e.g. if M1 and M3 have the same valency as M2 and Nb. Thepresence of induced oxygen vacancies provides the advantages discussedherein.

The precursor materials may include one or more metal oxides, metalhydroxides, metal salts or ammonium salts. For example, the precursormaterials may include one or more metal oxides or metal salts ofdifferent oxidation states and/or of different crystal structure.Examples of suitable precursor materials include but are not limited to:Nb₂O₅, Nb(OH)₅, Niobic Acid, NbO, Ammonium Niobate Oxalate, NH₄H₂PO₄,(NH₄)₂PO₄, (NH₄)₃PO₄, P₂O₅, H₃PO₃, Ta₂O₅, WO₃, ZrO₂, TiO₂, MoO₃, V₂O₅,ZrO₂, CuO, ZnO, Al₂O₃, K₂O, KOH, CaO, GeO₂, Ga₂O₃, SnO₂, CoO, Co₂O₃,Fe₂O₃, Fe₃O₄, MnO, MnO₂, NiO, Ni₂O₃, H₃BO₃, ZnO, and MgO. The precursormaterials may not comprise a metal oxide, or may comprise ion sourcesother than oxides. For example, the precursor materials may comprisemetal salts (e.g. NO₃—, SO₃—) or other compounds (e.g. oxalates,carbonates). For the substitution of the oxygen anion with otherelectronegative anions Q, the precursors comprising element Q mayinclude one or more organic compounds, polymers, inorganic salts,organic salts, gases, or ammonium salts. Examples of suitable precursormaterials comprising element Q include but are not limited to: melamine,NH₄HCO₃, NH₃, NH₄F, PVDF, PTFE, NH₄Cl, NH₄Br, NH₄I, Br₂, Cl₂, I₂,ammonium oxychloride amide, and hexamethylenetetramine.

Some or all of the precursor materials may be particulate materials.Where they are particulate materials, preferably they have a D₅₀particle diameter of less than 20 μm in diameter, for example from 10 nmto 20 μm. Providing particulate materials with such a particle diametercan help to promote more intimate mixing of precursor materials, therebyresulting in more efficient solid-state reaction during the heattreatment step. However, it is not essential that the precursormaterials have an initial particle size of <20 μm in diameter, as theparticle size of the one or more precursor materials may be mechanicallyreduced during the step of mixing said precursor materials to form aprecursor material mixture.

The step of mixing the precursor materials to form a precursor materialmixture and/or further precursor material mixture may be performed by aprocess selected from (but not limited to): dry or wet planetary ballmilling, rolling ball milling, high energy ball milling, high shearmilling, air jet milling, steam jet milling, planetary mixing, and/orimpact milling. The force used for mixing/milling may depend on themorphology of the precursor materials. For example, where some or all ofthe precursor materials have larger particle sizes (e.g. a D₅₀ particlediameter of greater than 20 μm), the milling force may be selected toreduce the particle diameter of the precursor materials such that thesuch that the particle diameter of the precursor material mixture isreduced to 20 μm in diameter or lower. When the particle diameter ofparticles in the precursor material mixture is 20 μm or less, this canpromote a more efficient solid-state reaction of the precursor materialsin the precursor material mixture during the heat treatment step. Thesolid state synthesis may also be undertaken in pellets formed at highpressure (>10 MPa) from the precursor powders.

The step of heat treating the precursor material mixture and/or thefurther precursor material mixture may be performed for a time of from 1hour to 24 hours, more preferably from 3 hours to 18 hours. For example,the heat treatment step may be performed for 1 hour or more, 2 hours ormore, 3 hours or more, 6 hours or more, or 12 hours or more. The heattreatment step may be performed for 24 hours or less, 18 hours or less,16 hours or less, or 12 hours or less.

The step of heat treating the precursor material mixture may beperformed in a gaseous atmosphere, preferably air. Suitable gaseousatmospheres include: air, N₂, Ar, He, CO₂, CO, O₂, H₂, NH₃ and mixturesthereof. The gaseous atmosphere may be a reducing atmosphere. Where itis desired to make an oxygen-deficient material, preferably the step ofheat treating the precursor material mixture is performed in an inert orreducing atmosphere.

The step of heat treating the further precursor material mixture isperformed under reducing conditions. Reducing conditions include underan inert gas such as nitrogen, helium, argon; or under a mixture of aninert gas and hydrogen; or under vacuum. Preferably, the step of heattreating the further precursor material mixture comprises heating underinert gas.

The further step of heat treating the mixed niobium oxide and/or themixed niobium oxide comprising element Q under reducing conditions maybe performed for a time of from 0.5 hour to 24 hours, more preferablyfrom 2 hours to 18 hours. For example, the heat treatment step may beperformed for 0.5 hour or more, 1 hours or more, 3 hours or more, 6hours or more, or 12 hours or more. The further step heat treating maybe performed for 24 hours or less, 18 hours or less, 16 hours or less,or 12 hours or less. Reducing conditions include under an inert gas suchas nitrogen, helium, argon; or under a mixture of an inert gas andhydrogen; or under vacuum. Preferably heating under reducing conditionscomprises heating under inert gas.

In some methods it may be beneficial to perform a two-step heattreatment. For example, the precursor material mixture and/or thefurther precursor material mixture may be heated at a first temperaturefor a first length of time, follow by heating at a second temperaturefor a second length of time. Preferably the second temperature is higherthan the first temperature. Performing such a two-step heat treatmentmay assist the solid-state reaction to form the desired crystalstructure. This may be carried out in sequence, or may be carried outwith an intermediate re-grinding step.

The method may include one or more post-processing steps after formationof the mixed niobium oxide. In some cases, the method may include apost-processing step of heat treating the mixed niobium oxide, sometimesreferred to as ‘annealing’. This post-processing heat treatment step maybe performed in a different gaseous atmosphere to the step of heattreating the precursor material mixture to form the mixed niobium oxide.The post-processing heat treatment step may be performed in an inert orreducing gaseous atmosphere. Such a post-processing heat treatment stepmay be performed at temperatures of above 500° C., for example at about900° C. Inclusion of a post-processing heat treatment step may bebeneficial to e.g. form deficiencies or defects in the mixed niobiumoxide, for example to induce oxygen deficiency; or to carry out anionexchange on the formed mixed niobium oxide e.g. N exchange for the Oanion.

The method may include a step of milling and/or classifying the mixedniobium oxide (e.g. impact milling, jet milling, steam jet milling, highenergy milling, high shear milling, pin milling, air classification,wheel classification, sieving) to provide a material with any of theparticle size parameters given above.

There may be a step of carbon coating the mixed niobium oxide to improveits surface electrical conductivity, or to prevent reactions withelectrolyte. This is typically comprised of combining the mixed niobiumoxide with a carbon precursor to form an intermediate material that maycomprise milling, preferably high energy milling. Alternatively or inaddition, the step may comprise mixing the mixed niobium oxide with thecarbon precursor in a solvent, such as water, ethanol or THF. Theserepresent efficient methods of ensuring uniform mixing of the mixedniobium oxide with the carbon precursor.

It has been found that a carbon precursor comprising polyaromatic sp²carbon provides a particularly beneficial carbon coating on mixedniobium oxides of the first aspect of the invention. Therefore, themethod of making a mixed niobium oxide may further comprise the stepsof: combining the mixed niobium oxide or the mixed niobium oxidecomprising element Q with a carbon precursor comprising polyaromatic sp²carbon to form an intermediate material; and heating the intermediatematerial under reducing conditions to pyrolyse the carbon precursorforming a carbon coating on the mixed niobium oxide and inducing oxygenvacancies in the mixed niobium oxide.

The intermediate material may comprise the carbon precursor in an amountof up to 25 wt %, or 0.1-15 wt %, or 0.2-8 wt %, based on the totalweight of the mixed niobium oxide and the carbon precursor. The carboncoating on the mixed niobium oxide may be present in an amount of up to10 wt %, or 0.05-5 wt %, or 0.1-3 wt %, based on the total weight of themixed niobium oxide. These amounts of the carbon precursor and/or carboncoating provide a good balance between improving the electronicconductivity by the carbon coating without overly reducing the capacityof the mixed niobium oxide by overly reducing the proportion of themixed niobium oxide. The mass of carbon precursor lost during pyrolysismay be in the range of 30-70 wt %.

The step of heating the intermediate material under reducing conditionsmay be performed at a temperature in the range of 400-1,200° C., or500-1,100° C., or 600-900° C. The step of heating the intermediatematerial under reducing conditions may be performed for a durationwithin the range of 30 minutes to 12 hours, 1-9 hours, or 2-6 hours.

The step of heating the intermediate material under reducing conditionsmay be performed under an inert gas such as nitrogen, helium, argon; ormay be performed under a mixture of an inert gas and hydrogen; or may beperformed under vacuum.

The carbon precursor comprising polyaromatic sp² carbon may be selectedfrom pitch carbons, graphene oxide, graphene, and mixtures thereof.Preferably, the carbon precursor comprising polyaromatic sp² carbon isselected from pitch carbons, graphene oxide, and mixtures thereof. Mostpreferably, the carbon precursor comprising polyaromatic sp² carbon isselected from pitch carbons. The pitch carbons may be selected from coaltar pitch, petroleum pitch, mesophase pitch, wood tar pitch, isotropicpitch, bitumen, and mixtures thereof.

Pitch carbon is a mixture of aromatic hydrocarbons of differentmolecular weights. Pitch carbon is a low cost by-product from petroleumrefineries and is widely available. The use of pitch carbon isadvantageous because pitch has a low content of oxygen. Therefore, incombination with heating the intermediate material under reducingconditions, the use of pitch favours the formation of oxygen vacanciesin the mixed niobium oxide.

Other carbon precursors typically contain substantial amounts of oxygen.For example, carbohydrates such as glucose and sucrose are often used ascarbon precursors. These have the empirical formula C_(m)(H₂O)_(n) andthus contain a significant amount of covalently-bonded oxygen (e.g.sucrose has the formula C₁₂H₂₂O₁₁ and is about 42 wt % oxygen). Thepyrolysis of carbon precursors which contain substantial amounts ofoxygen is believed to prevent or inhibit reduction of a mixed niobiumoxide, or even lead to oxidation, meaning that oxygen vacancies may notbe induced in the mixed niobium oxide.

Accordingly, the carbon precursor may have an oxygen content of lessthan 10 wt %, preferably less than 5 wt %.

The carbon precursor may be substantially free of sp³ carbon. Forexample, the carbon precursor may comprise less than 10 wt % sources ofsp³ carbon, preferably less than 5 wt % sources of sp³ carbon.

Carbohydrates are sources of sp³ carbon. The carbon precursor may befree of carbohydrates. It will be understood that some carbon precursorsused in the invention may contain impurities of sp³ carbon, for exampleup to 3 wt %.

The mixed niobium oxide of the first aspect of the invention maycomprise a carbon coating. Preferably the carbon coating comprisespolyaromatic sp² carbon. Such a coating is formed by pyrolysing a carbonprecursor comprising polyaromatic sp² carbon, preferably under reducingconditions, since the sp² hybridisation is largely retained duringpyrolysis. Typically, pyrolysis of a polyaromatic sp² carbon precursorunder reducing conditions results in the domains of sp² aromatic carbonincreasing in size.

Accordingly, the presence of a carbon coating comprising polyaromaticsp² may be established via knowledge of the precursor used to make thecoating. The carbon coating may be defined as a carbon coating formedfrom pyrolysis of a carbon precursor comprising polyaromatic sp² carbon.Preferably, the carbon coating is derived from pitch carbons.

The presence of a carbon coating comprising polyaromatic sp² carbon mayalso be established by routine spectroscopic techniques. For instance,Raman spectroscopy provides characteristic peaks (most observed in theregion 1,000-3,500 cm⁻¹) which can be used to identify the presence ofdifferent forms of carbon. A highly crystalline sample of sp³ carbon(e.g. diamond) provides a narrow characteristic peak at ˜1332 cm⁻¹.Polyaromatic sp² carbon typically provides characteristic D, G, and 2Dpeaks. The relative intensity of D and G peaks (I_(D)/I_(G)) can provideinformation on the relative proportion of sp² to sp³ carbon.

The mixed niobium oxide may have an I_(D)/I_(G) ratio as observed byRaman spectroscopy within the range of 0.85-1.15, or 0.90-1.10, or0.95-1.05.

X-ray diffraction may also be used to provide information on the type ofcarbon coating. For example, an XRD pattern of a mixed niobium oxidewith a carbon coating may be compared to an XRD pattern of the uncoatedmixed niobium oxide and/or to an XRD pattern of a pyrolysed sample ofthe carbon precursor used to make the carbon coating.

The carbon coating may be semi-crystalline. For example, the carboncoating may provide a peak in an XRD pattern of the mixed niobium oxidecentred at 2θ of about 260 with a width (full width at half maximum) ofat least 0.20°, or at least 0.25°, or at least 0.30°.

Examples

The mixed niobium oxides were synthesised by a solid-state route. In afirst step precursor materials (Nb₂O₅, GeO₂, ZnO, TiO₂, Cr₂O₃, Al₂O₃,Fe₂O₃, ZrO₂, and CuO) were milled to a D₅₀ (v/v) particle size below 20μm. The materials were mixed in stoichiometric proportions (50 g total)and combined in a homogeneous powder mixture by an impact mill at 20,000rpm. The resulting powders were heat treated in an alumina crucible in amuffle furnace in air at T₁=600-1350° C. for 0.5-24 h, providing thedesired Wadsley-Roth phase. Selected samples (9-11) were removed fromthe furnace after heat treatment, ground by impact mill at 20,000 rpm,and then had repeated heat treatment under similar conditions.Specifically, the precursor mixture was heated at a ramp rate of 5°/minto temperatures at or below 800° C., followed by a ramp rate of 1°/minto the maximum temperature for a holding period. An additional heattreatment step was also applied in some cases under a N₂ atmosphere atT₂=600-1350° C. for 0.5-12 h. For inclusion of anions, there was anadditional milling/mixing step with the precursor (PVDF in a 1:10 massratio for F; if N is required then C₃H₆N₆ in a 1:3 mass ratio versus theparent material may be used) prior to heat treatment in a N₂ or airatmosphere in one or two steps at T_(2a)/T_(2b)=300-1200° C. for 0.5-12h.

A final de-agglomeration step was utilised by impact milling or jetmilling to adjust to the desired particle size distribution wherenecessary. Specifically, the material was de-agglomerated by impactmilling at 20,000 RPM for 10 seconds. Particle Size Distributions wereobtained with a Horiba laser diffraction particle analyser for drypowder. Air pressure was kept at 0.3 MPa. The results are set out inTable 1.

TABLE 1 A summary of the materials synthesised. Particle sizedistribution has been evaluated by dry powder laser diffraction. TT_(2a) T_(2b) D10 D50 D90 Sample Material (° C.; h) (° C.; h) (° C.; h)(μm) (μm) (μm)  1* Zn₂Nb₃₄O₈₇ 1200; 12 — — 4.6 8.1 15.1  2**Zn₂Nb₃₄O_(87-x) 1200; 12 1200^(†); 5 — 4.4 7.4 12.3  3Ge_(0.1)Zn_(1.9)Nb₃₄O_(87.1)*** 1200; 12 — — 5.1 9.2 16.5  4Zn₂Nb₃₄O_(87-y)F_(y) 1200; 12 1200; 5^(†) 375; 24 4.0 6.8 11.3  5Cr_(0.4)Zn_(1.6)Ti_(0.4)Nb_(33.6)O₈₇ 1100; 12 — — 3.4 5.4 8.3  6Cr_(0.6)Zn_(1.4)Ti_(0.6)Nb_(33.4)O₈₇ 1100; 12 — — 3.5 5.4 8.5  7Cr_(0.8)Zn_(1.2)Ti_(0.8)Nb_(33.2)O₈₇ 1100; 12 — — 3.4 5.5 8.5  8Cr_(0.99)Zn_(1.01)Ti_(0.99)Nb_(33.01)O₈₇ 1100; 12 — — 3.4 5.5 8.4  9Cr_(0.4)Zn_(1.6)Zr_(0.4)Nb_(33.6)O₈₇ 1100; 12 × 3 — — 3.6 5.6 8.5 10Zn₂Fe_(0.2)Nb_(33.8)O_(86.8)*** 1100; 12 × 2 — — 3.8 5.8 8.8 11Al_(0.1)Zn_(1.9)Nb₃₄O_(87.05)*** 1100; 12 × 2 — — 4.0 6.1 9.2 12Cr_(0.99)Cu_(1.01)Ti_(0.99)Nb_(33.01)O₈₇ 1100; 12 — — 2.9 6.0 11.5*Comparative sample—unmodified ‘base’ Zn₂Nb₃₄O₈₇ **Induced oxygendeficiency may be calculated from e.g. TGA ***Oxygen stoichiometrycalculated assuming Ge⁴⁺, Zn²⁺, Fe³⁺, Al³⁺, and Nb⁵⁺ ^(†)This heattreatment step was carried out in a N₂ atmosphere. All others werecarried out in an air atmosphere.

Materials Characterisation

The phase purity of samples was analysed using a Rigaku Miniflex powderX-ray diffractometer in 2θ range (10-70°) at 1°/min scan rate.

FIG. 1 shows the measured XRD diffraction patterns for Samples 1-4, andFIG. 2 for Samples 5-12. Diffraction patterns have peaks at the samelocations (with some shift due to crystal modification, up to around0.2°), and match crystallography database entry JCPDS 28-1478. Certainsamples were found to be a phase mixture of monoclinic (JCPDS 28-1478,Reference a) and orthorhombic (PDF card: 04-021-7859, Reference b)crystal structures of the same Wadsley-Roth block structure(Zn₂Nb₃₄O₈₇), and so have been refined to this mixture. There is noamorphous background noise and the peaks are sharp and intense. Thismeans that all samples are crystalline, with crystallite size 45-55 nmaccording to the Scherrer equation and crystal structure matchingZn₂Nb₃₄O₈₇. This confirms the presence of a Wadsley-Roth crystalstructure.

TABLE 2 A summary table of unit cell parameters for each samplecalculated by Rietveld refinement of their powder XRD spectra withsoftware GSASII, and average crystallite size calculated by the Scherrerequation across the spectra. Crystallite Sample a [Å] b [Å] c [Å] β [°]Vol [Å³] χ² % Phase size [nm] Reference a 15.57 3.814 20.54 113.681117.02 — — — Reference b 28.709 3.826 20.624 90 2265.15 — — —  1*15.533 3.807 20.560 113.09 1118.61 9 100 a 48  2 15.537 3.806 20.563113.16 1118.09 10 100 a 49  3 15.560 3.823 20.649 113.16 1132.80 10 100a 48  4 15.542 3.808 20.562 113.15 1119.05 11 100 a 51  5a 15.594 3.82920.643 113.02 1134.41 10  34 a 48  5b 28.710 3.827 20.650 90 2269.10  66b  6a 15.594 3.828 20.635 113.04 1133.55 9  43 a 50  6b 28.703 3.82720.645 90 2267.69  57 b  7a 15.587 3.828 20.621 113.05 1132.23 9  40 a50  7b 28.693 3.827 20.635 90 2265.88  60 b  8a 15.584 3.828 20.612113.07 1131.35 9  41 a 52  8b 28.687 3.827 20.628 90 2264.52  59 b  9a15.606 3.830 20.650 113.07 1135.47 10  25 a 48  9b 28.725 3.828 20.65890 2271.51  75 b 10a 15.608 3.830 20.655 113.10 1135.74 10  12 a 48 10b28.726 3.828 20.664 90 2272.61  88 b 11a 15.608 3.827 20.648 113.111134.28 11  12 a 48 11b 28.722 3.829 20.667 90 2272.87  88 b 12 15.5663.831 20.610 113.12 1130.41 12 100 a 49 χ² represents the goodness offit and the accuracy of the Rietveld refinement.

Electrochemical Characterisation

Li-ion cell charge rate is usually expressed as a “C-rate”. A 1C chargerate means a charge current such that the cell is fully charged in 1 h,10C charge means that the battery is fully charged in 1/10th of an hour(6 minutes). C-rate hereon is defined from the reversible capacityobserved of the anode within the voltage limits applied in its secondcycle de-lithiation, i.e. for an anode that exhibits 1.0 mAh cm⁻²capacity within the voltage limits of 1.1-3.0 V, a 1C rate correspondsto a current density applied of 1.0 mA cm⁻². In a typical MNO materialas described herein, this corresponds to ˜200 mA/g of active material.

Electrochemical tests were carried out in half-coin cells (CR2032 size)for analysis. In half-coin tests, the active material is tested in anelectrode versus a Li metal electrode to assess its fundamentalperformance. In the below examples, the active material composition tobe tested was combined with N-Methyl Pyrrolidone (NMP), carbon black(Super P) acting as a conductive additive, and poly(vinyldifluoride)(PVDF) binder and mixed to form a slurry using a lab-scale centrifugalplanetary mixer. The non-NMP composition of the slurries was 92 wt %active material, 3 wt % conductive additive, 5 wt % binder. The slurrywas coated on an Al foil current collector to the desired loading of69-75 g m⁻² by doctor blade coating and dried by heating. The electrodeswere then calendared to a density of 2.6-2.9 g cm⁻³ at 80° C. to achievetargeted porosities of 35-40%. Electrodes were punched out at thedesired size and combined with a separator (Celgard porous PP/PE), Limetal, and electrolyte (1.3 M LiPF₆ in EC/DEC) inside a steel coin cellcasing and sealed under pressure. Cycling was then carried out at 23° C.at low current rates (C/10) for 2 full cycles of lithiation andde-lithiation between 1.1-3.0 V. Afterwards, the cells were tested fortheir performance at increasing current densities. During these tests,the cells were cycled asymmetric at 23° C., with a slow lithiation (C/5)followed by increasing de-lithiation rates (e.g. 1C, 5C, 10C) to providethe capacity retention, and nominal voltage at 5C. Nominal voltage vsLi/Li+ has been calculated from the integral of the V/Q curve divided bythe total capacity at 5C during de-lithiation. No constant voltage stepswere used. Data has been averaged from 5 cells prepared from the sameelectrode coating, with the error shown from the standard deviation.Accordingly, the data represent a robust study showing the improvementsachieved by the materials according to the invention compared to priormaterials. These data are shown in Tables 4 and 5.

Cell resistance has been calculated from the Direct Current InternalResistance (DCIR) of the half coin cell. In a typical measurement, thecell is lithiated to 100% State of Charge (SOC) and then delithiated to50% SOC at a rate of C/10, then after a rest of 0.5 h a 5C delithiationpulse is applied for 10 s, followed by another rest of 0.5 h. The DCIRis then calculated from V=IR, using the voltage immediately before thepeak from the pulse, and the measured maximum voltage during the pulse.

The electrical resistivity of the electrode coating was separatelyassessed by a 4-point-probe method with an Ossila instrument(T2001A3-UK) at 23° C. Slurries were formulated (the active materialcomposition to be tested was combined with N-Methyl Pyrrolidone (NMP),carbon black acting as a conductive additive, and poly(vinyldifluoride)(PVDF) binder and mixed to form a slurry using a lab-scale centrifugalplanetary mixer; the non-NMP composition of the slurries was 80 w. %active material, 10 w. % conductive additive, 10 w. % binder). Theslurry was then coated on a dielectric mylar film at a loading of 1mg/cm². Electrode-sized discs where then punched out and resistance ofthe coated-film was measured using a 4-point probe. The results forsheet resistance (Ω/square) are outlined in Table 3, with error based onthe standard deviation of 3 measurements.

Homogeneous, smooth coatings on both Cu and Al current collector foils,the coatings being free of visible defects or aggregates may also beprepared as above for these samples with a centrifugal planetary mixerto a composition of up to 94 wt % active material, 4 wt % conductiveadditive, 2 wt % binder. These can be prepared with both PVDF (i.e.NMP-based) and CMC:SBR-based (i.e. water-based) binder systems. Thecoatings can be calendared at 80° C. for PVDF and 50° C. for CMC:SBR toporosities of 35-40% at loadings from 1.0 to 5.0 mAh cm⁻². This isimportant to demonstrate the viability of these materials in both highenergy and high-power applications, with high active material content.

TABLE 3 Summary of 4-point probe resistivity measurement results SheetResistivity Sample [Ω/square]  1* 1242 ± 156 2 1027 ± 13  3 1041 ± 103

TABLE 4 A summary of electrochemical testing results from Li-ion halfcoin cells. In general it is beneficial to have a higher capacity, ahigher Coulombic efficiency, and a lower nominal voltage. DelithiationCoulombic Nominal specific efficiency de-lithiation capacity 2^(nd)1^(st) cycle voltage at C/10 cycle at C/10 5 C vs Li/Li⁺ Sample [mAh/g][%] [V]  1* 196 ± 1 97.84 ± 0.02 1.836 ± 0.008 2 197 ± 1 98.18 ± 0.541.816 ± 0.010 3 201 ± 1 98.08 ± 0.09 1.776 ± 0.013 4 200 ± 4 97.76 ±0.15 1.859 ± 0.028 5 206 ± 1 99.09 ± 0.64 1.820 ± 0.007 6 208 ± 3 98.69± 0.11 1.821 ± 0.002 7 213 ± 2 98.82 ± 0.31 1.799 ± 0.001 8 212 ± 298.87 ± 0.27 1.795 ± 0.003 9 202 ± 1 98.46 ± 0.15 1.787 ± 0.001 10  201± 3 98.27 ± 0.32 1.818 ± 0.004 11  205 ± 1 98.51 ± 0.51 1.813 ± 0.00412  208 ± 1 93.20 ± 0.07 1.827 ± 0.002 *Comparative sample

TABLE 5 A summary of further electrochemical testing results from Li-ionhalf coin cells. Delithiation Delithiation Delithiation specificspecific specific capacity 1 C capacity 5 C capacity 10 C Sample [mAh/g][mAh/g] [mAh/g]  1* 190 ± 2 186 ± 2 180 ± 4 2 192 ± 2 191 ± 2 184 ± 3 3195 ± 1 192 ± 1 184 ± 2 4 194 ± 4 191 ± 4 180 ± 7 5 202 ± 2 200 ± 2 192± 1 6 202 ± 2 198 ± 2 191 ± 2 7 205 ± 4 203 ± 3 194 ± 2 8 206 ± 1 202 ±1 197 ± 1 9 197 ± 2 194 ± 2 190 ± 2 10  193 ± 2 191 ± 2 188 ± 2 11  198± 3 196 ± 3 192 ± 3 12  203 ± 1 202 ± 1 195 ± 1 *Comparative sample

TABLE 6 A summary of further electrochemical testing results from Li-ionhalf coin cells De-lithiation De-lithiation De-lithiation capacitycapacity capacity retention retention retention Cell 1 C/0.5 C 5 C/0.5 C10 C/0.5 C resistance Sample [%] [%] [%] [mOhms]  1* 99.6 ± 0.3 97.5 ±0.6 94.1 ± 2.1 21.7 ± 0.8 2 99.6 ± 0.1 98.7 ± 0.2 95.3 ± 1.3 18.8 ± 0.73 99.9 ± 0.1 98.2 ± 0.3 94.1 ± 1.0 18.4 ± 1.1 4 99.6 ± 0.1 98.3 ± 0.992.5 ± 2.5 23.2 ± 5.7 5 99.9 ± 0.1 98.7 ± 0.1 94.9 ± 0.5 18.0 ± 1.0 699.5 ± 0.1 97.7 ± 0.3 94.4 ± 0.7 19.6 ± 1.1 7 99.7 ± 0.1 98.6 ± 0.3 94.2± 1.0 17.0 ± 0.6 8 99.6 ± 0.1 98.5 ± 0.1 96.0 ± 0.6 18.8 ± 1.1 9 99.6 ±0.2 98.0 ± 0.4 95.9 ± 0.3 19.6 ± 0.4 10  99.7 ± 0.1 98.7 ± 0.4 96.7 ±0.5 21.5 ± 1.4 11  99.6 ± 0.1 98.4 ± 0.4 96.5 ± 0.5 20.1 ± 1.6 12  99.9± 0.1 99.5 ± 0.1 96.2 ± 0.5 20.6 ± 0.8 *Comparative sample

Discussion

The mixed niobium oxide Sample 1* has been modified through a cationsubstitution approach in Sample 3, focused at the Zn²⁺ cationssubstituted by Ge⁴⁺. In Samples 5-8, Zn²⁺ cations have been substitutedby Cr³⁺ cations and Nb⁵⁺ cations have been substituted by Ti⁴⁺ cations,spanning a wide range of the variables a and b. Sample 10 substitutesNb⁵⁺ by Fe³⁺. Sample 11 substitutes Zn²⁺ by Al³⁺. Sample 12 is based onCu₂Nb₃₄O₈₇ where Cu²⁺ cations have been substituted by Cr³⁺ cations andNb⁵⁺ cations have been substituted by Ti⁴⁺ cations. Increased valencymay be compensated for by partial oxygen excess (i.e. c<0) and/orpartial reduction of Nb⁵⁺. Decreased vacancy may be compensated for bythe formation of oxygen vacancies (i.e. c>0). These modifications areexpected to provide an advantage versus the base crystal structure ofSample 1* through the combination of (a) altered ionic radii, (b)altered valency, and (c) altered voltage. Altered ionic radii can giverise to beneficial changes in electrochemical performance due tochanging unit cell size and local distortions in crystal structurealtering available lithiation sites or lithiation pathways—potentiallyimproving Coulombic efficiency, capacity, performance at high rate, andlifetime. Altered valency provides significantly improved electricalconductivity of the material due to providing available intermediateenergy levels for charge carriers. These effects are shown by the lowerresistivity observed in Table 3 of the modified samples vs. Sample 1*,and by the improvements in specific capacity, coulombic efficiency,de-lithiation voltage at 5C, and capacity retention at 1C, 5C, and 10Cobserved in Tables 4-6. These are key results demonstrating the utilityof the modified mixed niobium oxides according to the invention for usein high-power Li-ion cells designed for fast charge/discharge.

Table 2 demonstrates the alterations in unit cell parameters observedupon cation exchange, observed due to alterations of ionic radii andelectronic structure of these materials.

It is expected that similar benefits will be observed with the describedcation exchange approach for this material for use in Li-ion cells.

The mixed niobium oxide Sample 1* has modified through the introductionof induced oxygen deficiency by a heat treatment in an inert or reducingatmosphere to provide Sample 2. By treating the ‘base’ oxide at hightemperature in an inert or reducing atmosphere it may be partiallyreduced, and maintain this upon return to room temperature and exposureto an air atmosphere. This is accompanied with an obvious colour change,for example Sample 2 is grey/black in colour vs white for Sample 1*.This colour change demonstrates a significant change in the electronicstructure of the material, allowing it to interact with differentenergies (i.e. wavelength) of visible light due to the reduced band gap.This is reflected in sample 2, demonstrating an improved delithiationvoltage at a rate of 5C, which corresponds to a reduced level ofpolarisation in the cell.

The induced oxygen deficiency results in a defect in the crystalstructure, e.g. where an oxygen anion has been removed, and the overallredox state of the cations is reduced in turn. This provides additionalenergetic states improving material electrical conductivitysignificantly, and alters the band gap energy as demonstrated by colourchanges. This is shown by the lower resistivity observed in Table 3 forSample 2 vs. Sample 1*. If induced oxygen deficiency is present beyond 5atomic % (i.e. c>4.35), then the crystal structure may be less stable.

The mixed niobium oxide Sample 1* has modified through anionsubstitution (O²⁻ by F⁻) to provide Sample 4. Improvements in specificcapacity were observed (Tables 4 and 5).

It is expected that similar benefits will be observed in any of thedescribed mixed niobium oxides utilising any combination of M1, M2, M3,Q, a, b, c, and d within the described limits for use in Li ion cells.

Mixtures with LTO

A modified mixed niobium oxide was tested as an active electrodematerial in combination with a commercial material, demonstrating theutility of the modified mixed niobium oxides for incorporation into andimprovement of existing battery technologies.

Commercial-grade LTO (Li₄Ti₅O₁₂) was purchased from Targray TechnologyInternational Inc with properties outlined in Table E1 (Sample E1). TheWadsley-Roth material was synthesised in-house by a solid-state route.In a first step precursor materials (Nb₂O₅, Cr₂O₃, ZnO, and TiO₂) weremixed in stoichiometric proportions (200 g total) and ball-milled at 550rpm with a ball to powder ratio of 10:1 for 3 h. The resulting powderwas heat treated in an alumina crucible in a muffle furnace in air atT₁=1100° C. for 12 h, providing the desired Wadsley-Roth phase.

Active electrode material mixtures of MNO and LTO were obtained by lowto high energy powder mixing/blending techniques, such as by rotationalmixing in multiple directions, rotational V-type blending over a singleaxis, planetary mixing, centrifugal planetary mixing, high shear mixing,and other typical mixing/blending techniques. In this case, mixing wasachieved with a centrifugal planetary mixer on 5 g batches of materials,mixed at 2000 rpm for 3 mins, 10 times.

TABLE E1 A summary of the materials utilised. Particle size distributionhas been evaluated by dry powder laser diffraction, and surface area bythe BET method using N₂. BET Surface D10 D50 D90 Area Sample Material(μm) (μm) (μm) [m² g⁻¹] E1 LTO (from commercial supplier) 0.8 2.5 5.016.0* E2 Cr_(0.4)Zn_(1.6)Ti_(0.4)Nb_(33.6)O₈₇ 3.3 5.3 8.3 0.6 *Frommanufacturer specification sheet.

Materials Characterisation

The phase purity of samples was analysed using a Rigaku Miniflex powderX-ray diffractometer in 2θ range (20-70°) at 1°/min scan rate. Thediffraction pattern for Sample E1 matches JCPDS crystallography databaseentry JCPDS 49-0207, which corresponds to the spinel crystal structureof Li₄Ti₅O₁₂. There is no amorphous background noise and the peaks aresharp and intense. This means that the sample is crystalline, withcrystallite size 43±7 nm according to the Scherrer equation. Thediffraction pattern for Sample E2 confirms the presence of the desiredWadsley-Roth crystal structure Zn₂Nb₃₄O₈₇. There is no amorphousbackground noise and the peaks are sharp and intense. This means thatthe sample is phase-pure and crystalline, with crystallite size 49±6 nmaccording to the Scherrer equation.

Particle Size Distributions were obtained with a Horiba laserdiffraction particle analyser for dry powder. Air pressure was kept at0.3 MPa. The results are set out in Table E1. BET surface area analysiswas carried out with N₂ on a BELSORP-miniX instrument at 77.35 K and areset out in Table E1.

Electrochemical Characterisation

Electrochemical tests were carried out in half-coin cells (CR2032 size)for analysis. There are some differences to the testing methodology usedfor Samples 1-12 above, meaning that the results may not be directlycomparable. In half-coin tests, the active material is tested in anelectrode versus a Li metal electrode to assess its fundamentalperformance. In the below examples, the active material composition tobe tested was combined with N-Methyl Pyrrolidone (NMP), carbon blackacting as a conductive additive, and poly(vinyldifluoride) (PVDF) binderand mixed to form a slurry using a lab-scale centrifugal planetarymixer. The non-NMP composition of the slurries was 90 wt % activematerial, 6 wt % conductive additive, 4 wt % binder. The slurry wascoated on an Al foil current collector to the desired loading of 5.7-6.5mg cm⁻² by doctor blade coating and dried. The electrodes were thencalendared to a density of 2.00-3.75 g cm⁻³ (dependent on materialdensity) at 80° C. to achieve targeted porosity of 35-40%. Porosity wascalculated as the measured electrode density divided by the weightedaverage density of each component of the composite electrode coatingfilm. Electrodes were punched out at the desired size and combined witha separator (Celgard porous PP/PE), Li metal, and electrolyte (1.3 MLiPF₆ in EC/DEC) inside a steel coin cell casing and sealed underpressure. Cycling was then carried out at low current rates (C/10) for 2full cycles of lithiation and de-lithiation between 1.1-3.0 V.Afterwards, the cells were tested for their performance at increasingcurrent densities. During rate tests, the cells were cycled asymmetric,with a slow lithiation (C/5, with a CV step at 1.1V to C/20 currentdensity) followed by increasing de-lithiation rates for de-lithiationrate tests. All electrochemical tests were carried out in a thermallycontrolled environment at 23° C.

The first cycle efficiency was calculated as the fraction ofde-lithiation capacity/lithiation capacity in the first cycle at C/10.The nominal voltage at each C-rate was determined by integrating thevoltage-capacity curves and then by dividing it by the total capacity.

To quantify the significance of the differences in data observed, anerror calculation was carried out and applied to the values for specificcapacity. The error for these was approximated as the largest errorpossible with the microbalance used (±0.1 mg), and the lowest loadingelectrode (5.7 mg cm⁻²) on a 14 mm electrode disc. This provides anerror of ±1.1%, which has been applied to each capacity measurement.Error in Coulombic efficiency, capacity retention, and voltage wereassumed to be negligible for the cell tested as the instrument accuracyfar exceeds the stated significant figures, and the values areindependent of the balance errors.

TABLE E2 A summary of the electrochemical tests undertaken with SamplesE1 and E2. Achieved electrode conditions are also referenced for eachtest, providing smooth electrodes free of agglomerates, that demonstrategood adhesion and cohesion to the current collector. Test Ref. A* B CContent of Sample 100 — 50 E1 [w/w %] Content of Sample — 100 50 E2 [w/w%] Electrode loading 6.3 6.3 6.3 [mg cm⁻²]

TABLE E3 A summary of electrochemical testing results from Li-ion halfcoin cells. De-lithiation Initial specific coulombic capacity C/10efficiency Test [mAh/g] [%] A* 161 ± 2 96.86 B 213 ± 2 98.30 C 185 ± 297.63

TABLE E4 A summary of electrochemical testing results at increasingcurrent densities from Li-ion half coin cells. 1 C/0.5 C 2 C/0.5 C 5C/0.5 C 10 C/0.5 C de-lithiation de-lithiation de-lithiationde-lithiation capacity capacity capacity capacity retention retentionretention retention Test [%] [%] [%] [%] A* 99.4 98.8 97.5 96.3 B 99.198.6 97.2 89.8 C 100.0 98.9 97.4 93.1

TABLE E5 A summary of the nominal de-lithiation voltage at each C-rate.Nominal De-lithiation Voltage vs Li/Li⁺ [V] Test 0.1 C 0.5 C 2 C 5 C 10C A* 1.57 1.59 1.61 1.67 1.78 B 1.58 1.60 1.69 1.84 2.09 C 1.58 1.601.68 1.81 2.03

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention. For the avoidance of any doubt,any theoretical explanations provided herein are provided for thepurposes of improving the understanding of a reader. The inventors donot wish to be bound by any of these theoretical explanations. Anysection headings used herein are for organizational purposes only andare not to be construed as limiting the subject matter described.

1. An active electrode material comprising a mixed niobium oxide,wherein the mixed niobium oxide has the compositionM1_(a)Zn_(2-a)M3_(b)Nb_(34-b)O_(87-c), wherein: M1 is selected from Mg,V, Cr, Mo, W, Fe, Cu, Al, Ge, and mixtures thereof; M3 is selected fromTi, Zr, V, Cr, Mo, W, Fe, Cu, Zn, Al, P, and mixtures thereof; 0<a<1.0;0<b≤3.4; −0.5≤c≤4.35.
 2. The active electrode material of claim 1,wherein 0<a≤0.6.
 3. The active electrode material of claim 1, wherein0<b≤1.5.
 4. The active electrode material of claim 1, wherein c≠0. 5.The active electrode material of claim 1, wherein 0≤c≤4.35.
 6. Theactive electrode material of claim 1, wherein M1 has an equal or highervalency than 2+.
 7. The active electrode material of claim 1, wherein M3has a lower valency than 5+.
 8. The active electrode material of claim1, wherein M3 does not comprise Zn and/or Cu.
 9. The active electrodematerial of claim 1, wherein: M1 is selected from Cr, Al, Ge, andmixtures thereof; M3 is selected from Ti, Zr, Fe, and mixtures thereof0<a<1.0; 0<b≤1.5; −0.5≤c≤4.35.
 10. The active electrode material of aclaim 1, wherein: M1 is Cr; M3 is selected from Ti, Zr, and mixturesthereof; 0.01<a<1.0; 0.01<b<1.0; −0.5≤c≤2.
 11. The active electrodematerial of claim 1, wherein the mixed niobium oxide is oxygendeficient.
 12. The active electrode material of claim 1, wherein themixed niobium oxide is coated with carbon.
 13. The active electrodematerial of claim 1, wherein the mixed niobium oxide is in particulateform and has a D₅₀ particle diameter in the range of 0.1-100 μm. 14.-20.(canceled)
 21. The active electrode material of claim 1, wherein themixed niobium oxide has a BET surface area in the range of 0.1-100 m²/g.22. The active electrode material of claim 1, wherein the mixed niobiumoxide further comprises Li and/or Na.
 23. The active electrode materialof claim 1, wherein the crystal structure of the mixed niobium oxide asdetermined by X-ray diffraction corresponds to the crystal structure ofZn₂Nb₃₄O₈₇.
 24. The active electrode material of claim 1 comprising themixed niobium oxide and at least one other component selected from abinder, a solvent, a conductive additive, a different active electrodematerial, and mixtures thereof.
 25. (canceled)
 26. An electrodecomprising the active electrode material of claim 1 in electricalcontact with a current collector.
 27. A lithium-ion battery or asodium-ion battery comprising an anode, a cathode, and an electrolytedisposed between the anode and the cathode, wherein the anode comprisesthe active electrode material of claim
 1. 28. (canceled)
 29. A method ofmaking the mixed niobium oxide of claim 1, the method comprising stepsof: providing one or more precursor materials; mixing said precursormaterials to form a precursor material mixture; and heat treating theprecursor material mixture in a temperature range from 400° C.-1350° C.,thereby providing the mixed niobium oxide. 30.-32. (canceled)